Congestive heart failure (CHF) is a disease affecting approximately 2% of the population of the United States (Sami, M. H. [1991] J. Clin. Pharmacol. 31:1081). Despite advances in the diagnosis and treatment of CHF, the prognosis remains poor with a 5-year mortality rate higher than 50% from the time of diagnosis (McFate Smith, W. [1985] Am. J. Cardiol. 55:3A; McKee, P. A., W. P. Castelli, P. M. McNamara, W. B. Kannel [1971] N. Engl. J. Med. 285:1441). In patients with CHF, the rate of survival is lowest in those patients with severe depression of left ventricular function and patients who have frequent ventricular arrhythmias. Patients with ventricular arrhythmias and ischemic cardiomyopathy have an increased risk of sudden death. The presence of ventricular tachycardia in patients with severe CHF results in a three-fold increase in sudden death compared to those without tachycardia (Bigger, J. T., Jr. [1987] Circulation 75(suppl.IV):28). Because of the high prevalence of sudden unexpected death in patients with CHF, there has been a growing interest in the prognostic significance of arrhythmias in these patients.
Several compounds have been used in the management of cardiac arrhythmias in patients with congestive heart failure. Unfortunately, anti-arrhythmic drug therapy has been disappointing. The efficacy of anti-arrhythmic drugs markedly decreases as left ventricular function declines, such that only a small fraction of patients with CHF are responsive to anti-arrhythmic therapy. No anti-arrhythmic drug has prevented sudden death in patients with CHF and there is even a question of increased mortality associated with certain anti-arrhythmic drugs (the CAST investigators [1989] N. Engl. J. Med. 321:406).
Scientists define tachycardia and ventricular fibrillation as being of multiple nature. It now seems clear, and is accepted in the art, that re-entry is the underlying mechanism to most sustained arrhythmias. Prolonging ventricular repolarization as a means of preventing ventricular arrhythmias has consequently received renewed attention. This points to Class-III agents as drugs of choice in the treatment of arrhythmias. A Class-III agent, as referred to herein, is an agent which is classified as such in the Vaughan-Williams classification of anti-arrhythmic drugs. A Class-III agent exerts its primary anti-arrhythmic activity by prolonging cardiac action potential duration (APD), and thereby the effective refractory period (ERP), with no effect on conduction. These electrophysiological changes, which are brought about by blockade of cardiac potassium channels, are well known in the art. Because the blockade of cardiac potassium channels is not associated with depression of the contractile function of the heart, Class-III agents are particularly attractive for use in patients with CHF. Unfortunately, the existing Class-III agents are limited in their utility by additional pharmacological activities, lack of good oral bioavailability, or a poor toxicity profile. Two Class III agents currently marketed are bretylium (i.v. only) and amiodarone (i.v. and p.o.).
Amiodarone is an anti-arrhythmic agent with complex electrophysiological activity including Class-I (sodium channel), Class-II (beta-receptor), Class-III (potassium channel), and even Class-IV (calcium channel) properties, thus acting on both cardiac conduction and cardiac repolarization parameters (Charlier et al., [1969] Cardiologia, 54:82; Singh et al., [1970] Br. J. Pharmacol. 39:657; Rosenbaum et al., [1974] Am. J. Cardiol. 34:215; Rosenbaum et al., [1976] Am. J. Cardiol. 38:934). The corresponding EKG effects are reduction in heart rate (HR) and prolongation of the PR, QRS and QT intervals (Naccarelli et al., [1985] Pharmacotherapy, 6:298). Because of these combined electrophysiological properties, amiodarone is effective against ventricular and supra-ventricular arrhythmias, including atrial fibrillation and flutter, paroxysmal supraventricular tachycardia, ventricular premature beats (VPB), sustained and non-sustained ventricular tachycardia (VT), and ventricular fibrillation (VF) (Naccarelli et al., [1985] Pharmacotherapy, 6:298; Kerr et al., [1996], In Cardiovascular Drug Therapy, 2nd ed., Editor: Messerli, F. H. W. B. Saunders Co. pp. 1247-1264).
Amiodarone is one of the very few drugs that actually reduce mortality rates in high-risk patients (post-myocardial infarction patients and patients with congestive heart failure)(Cairns et al., [1997] Lancet, 349:675; Julian et al., [1997] Lancet, 349:667; Amiodarone Trials Meta-Analysis Investigators: Effect of Prophylactic Amiodarone on Mortality After Acute Myocardial Infarction and in Congestive Heart Failure: Meta-Analysis of Individual Data from 6500 Patients in Randomised Trials. Lancet, 1997, 350, 1417-24). Unfortunately, because of its life-threatening side-effects and the substantial management difficulties associated with its use, amiodarone is indicated only for life-threatening recurrent ventricular arrhythmias when these have not responded to documented adequate doses of other available anti-arrhythmics or when alternative agents are not tolerated (Vrobel et al., [1989] Progr. In Cardiovasc. Dis., 31:393). The pharmacokinetic properties of amiodarone are characterized by slow absorption, moderate bioavailability, high lipophilicity, and a very large volume of distribution (60 L/kg on average). Its elimination is almost exclusively hepatic and its clearance rate is very slow. Its terminal elimination half-life is 53 days (Naccarelli et al., [1985] Pharmacotherapy, 6:298). As a consequence, upon long-term administration, amiodarone accumulates in virtually every organ including poorly perfused tissues such as the lens. The onset of its anti-arrhythmic activity may take days, or even weeks to appear. The onset of activity can be shortened with the administration of intravenous loading doses, but is still too long (Kowey et al., [1995] Circulation, 92:3255). Cardioprotective agents and methods which employ amiodarone in synergistic combination with vasodilators and beta blockers have been described for use in patients with coronary insufficiency (U.S. Pat. No. 5,175,187). Amiodarone has also been described for reducing arrhythmias associated with CHF as used in combination with anti-hypertensive agents, e.g., (S)-1-[6-amino-2-[[hydroxy(4-phenylbutyl)phosphinyl]oxyl]-L-proline (U.S. Pat. No. 4,962,095) and zofenopril (U.S. Pat. No. 4,931,464). However, amiodarone is a difficult drug to manage because of its numerous side effects, some of which are serious.
Amiodarone has several potentially fatal toxicities, the most important of which is pulmonary toxicity (hypersensitivity pneumonitis or interstitial/alveolar pneumonitis). Pulmonary toxicity is reversible if the progression of the symptoms is recognized on time, but is still fatal 10% of the time (Kerr et al., [1996], In Cardiovascular Drug Therapy, 2nd ed., Editor: Messerli, F. H. W. B. Saunders Co. pp. 1247-1264; Vrobel et al., [1989] Progr. In Cardiovasc. Dis., 31:393). Liver injury is also common but usually mild, although liver disease can occur and has been fatal in some cases. Even though toxicity is usually reversible upon cessation of drug administration, the real danger with amiodarone comes from its slow kinetics, especially slow elimination. For example, although the frequency of pro-arrhythmic events associated with amiodarone appears to be less than with other anti-arrhythmic agents (2 to 5%), the effects are prolonged when they occur. Even in patients at high risk of sudden death, in whom the toxicity of amiodarone is an acceptable risk, amiodarone poses major management problems that could be life-threatening, so every effort is made to utilize alternative agents first.
The most serious long-term toxicity of amiodarone derives from its kinetics of distribution and elimination. It is absorbed slowly, with a low bioavailability and relatively long half-life. These characteristics have clinically important consequences, including the necessity of giving loading doses, a delay in the achievement of full anti-arrhythmic effects, and a protracted period of elimination of the drug after its administration has been discontinued.
Amiodarone can also interact negatively with numerous drugs including aprindine, digoxin, flecainide, phenytoin, procainamide, quinidine, and warfarin. It also has pharmacodynamic interactions with catecholamines, diltiazem, propranolol, and quinidine, resulting in alpha- and beta-antagonism, sinus arrest and hypotension, bradycardia and sinus arrest, and torsades de pointes and ventricular tachycardias, respectively. There is also evidence that amiodarone depresses vitamin K-dependent clotting factors, thereby enhancing the anticoagulant effect of warfarin.
Numerous adverse effects limit the clinical applicability of amiodarone. Important side effects can occur including corneal microdeposits, hyperthyroidism, hypothyroidism, hepatic dysfunction, pulmonary alveolitis, photosensitivity, dermatitis, bluish discoloration, and peripheral neuropathy.
There is no Class-III agent presently marketed that can be used safely in patients with CHF. The cardiovascular drug market is the largest in any field of drug research, and an effective and safe Class-III anti-arrhythmic agent useful in patients with CHF is expected to be of substantial benefit. Therefore, a drug which could successfully improve the prognosis of CHF patients, but with a safety profile much improved over that of amiodarone, would be extremely useful and desired.
U.S. Pat. Nos. 5,364,880; 5,440,054; and 5,849,788 (all to Druzgala) disclose novel anti-arrhythmic amiodarone analogs which are metabolized by esterases. The 2-butyl chain of amiodarone was functionalized to include an ester moiety, thus allowing endogenous esterases to metabolize the compounds into a primary metabolite containing a carboxylic acid moiety. Advantages associated with these compounds include smaller distribution volumes, shorter onset of activity, faster elimination rates, and safer long-term toxicity profiles. These amiodarone derivatives were synthesized as racemic mixtures.
The observed pharmacological activity of a given compound is the result of a complex interaction between its intrinsic activity at receptor level, its physical properties that determine transport through biological membranes, and its affinity toward metabolizing enzymes. As a result of this, it is practically impossible to predict differences in pharmacological activities between compounds that have very similar structures and similar physicochemical properties, such as optical isomers.
Biological systems however, because they are made of an assemblage of chiral subunits, are capable of recognizing optical isomers. The direct consequence of this chirality is often expressed by differences in receptor affinity, resulting in widely different pharmacological activities between optical isomers of the same drug. One of the most striking examples in the anti-arrhythmic field is the difference in pharmacological activity between d- and l-sotalol. Whereas d-sotalol is a class-III anti-arrhythmic, l-sotalol is a beta-blocker that is devoid of class-III properties. Clinical trials have demonstrated that neither d- nor l-sotalol alone are sufficient for efficient anti-arrhythmic activity in man, but that the mixture d,l-sotalol is required.
The subject invention pertains to novel enantiomerically pure compounds, and compositions comprising the compounds, for the treatment of cardiac arrhythmias. The subject invention further concerns a method of making and purifying the compounds. The isolated enantomerically purified compounds and compositions of these compounds exhibit unexpectedly distinct and advantageous characteristics, such as a markedly superior ability to reduce or inhibit ventricular premature beats, as compared to racemic mixtures of the compounds.
The enantiomerically pure compounds of the subject invention show differences at the kinetic and the dynamic levels that were totally unpredictable a priori. These pharmacological properties provide for the ability to reduce undesirable side effects associated with anti-arrhythmic drugs while maintaining a superior ability to modulate cardiac function in treatment regimens.